Harmonics in the woods

By BILL CHAISSON
Of a Feather
When I was learning biology in middle school, I was introduced to the concept of neoteny, the retention of juvenile characteristics into adulthood (something I sort of looked forward to). The example the teachers/textbooks provided was almost invariably the mudpuppy or the axolotl. Mudpuppies are aquatic North American salamanders and the axolotl is more obscure Mexican salamander, but both retain gills into adulthood and remain fully aquatic.

The average American student is much less likely to encounter either of these animals than they are to see a juvenile robin or bluebird or an adult hermit thrush. As a teacher, I always tried to “make it real” for the students. When the examples are too obscure, their natural (and healthy) skepticism resists absorption of an abstract concept like neoteny. For a lot of students, like it or not, seeing is believing.

The genus Catharus includes all the brown- to olive-gray-backed thrushes with dark speckled (or streaked) off-white chests and sides. (All except the wood thrush, which is in its own genus, Hylocichla.) The juvenile plumage — the first contour feathers after the natal down — of robins and bluebirds is a brown or gray back and a speckled or streaked chest and sides. In late summer robins and bluebirds undergo a partial molt and shed everything except their primary, secondary and tail feathers emerging as birds very similar to the adults.

The adult plumage of Catharus thrushes is not by any means identical to juvenile plumage of the robin or bluebird, but the resemblance is undeniable. But while the sexual development of robins and bluebirds is accompanied by a clear change in plumage, the Catharus thrushes become sexually mature while retaining a juvenile appearance.

When development proceeds at varying rates, the process is called heterochrony, a word with Greek roots that translates roughly as “different timing.” The retention of juvenile traits into adulthood causes heterochrony, specifically paedomorphosis. When you are lobbing a lot of $64 words like this at students, it is judicious to offer examples they are likely to see and hear, rather than, say, a fully aquatic amphibian that requires patience to find and a net to catch.

Robins and bluebirds live in our backyards, and robins in particular are prone to building nests in hanging plants on our front porches or in a large shrub next to the driveway. Which is to say, seeing juvenile robins and bluebirds is no big deal. Experiencing the Catharus thrushes, however, might require a little more effort. But not much.

These conservative looking birds include some of the more stunning singers in the avian pantheon. In the northeastern United States we have four species: the veery, and the hermit, Swainson’s, and Bicknell’s thrushes. The first two require the shortest excursions to find.

The veery has an onomatopoetic name; its song is a downward spiraling flute-like melody that consists of paired notes repeated in descending pitch. It is a simple song on the face of it, but as you listen more closely you will realize that it is double-timbred; the bird is singing a note and a sort of grace note … simultaneously. So, while the dominant notes are “VEE-rrr, VEE-rrr, VEE-rrr,” they have a dreamy, slurred quality produced by their syrinx. The organ is named for the Greek word for the panpipes, which are played by blowing over two openings at the same time.

While veeries are birds of wet woodlands on valley floors, hermit thrushes live in coniferous forests on the lower slopes of mountains. The veery is a rich red-brown from crown to tail; only the tail of the hermit thrush is rufous. And while the chest of the hermit thrush is distinctly spotted, the veery’s is lightly spotted to streaked and dusted with cinnamon.

The singing of the hermit thrush is more complex than the veery’s and even more haunting. The hermit thrush sounds beautiful to us because its song follows the mathematical rules that Western culture associates with harmony. In this scheme the intervals between notes are multiples of the frequency of the basic note that begins the melody; this is called a harmonic series. Claims by ornithologists that hermit thrushes sing in harmonic series were not initially believed by musicologists. But slowing down recordings of the birds’ singing showed that a majority of their songs—and each male bird sings six to 10 different ones—are in harmonic series.

In addition to this adherence to harmonic pitch intervals, the hermit thrush song is full of dramatic pauses that can literally make you hold your breath. And this is a bird that breeds, like the veery, literally throughout the state of New Hampshire.

Anyone can stand at the edge of a wet deciduous woodland or a upland coniferous forest at dusk and listen to the veery and hermit thrush sing. Seeing them is a little harder. Both birds forage in the understory and midstory, looking for insects and spiders. Their brownish backs and speckled breasts are good camouflage against a backdrop of dappled sunlight, leaf litter, foliage, and the thin, criss-crossing branches of understory trees and shrubs. But they tend to sit still while they sing and to sit in slightly more exposed positions.

The Swainson’s thrush dwells in higher altitude coniferous forests and is present in New Hampshire only from Grafton County north. Its song is the reverse of the veery’s; it spirals upward and includes more notes, but lacks the dramatic slowness and harmonic integrity of the hermit thrush.

The Bicknell’s thrush has only recently been separated from the gray-cheeked. It was once considered to be the scattered southern population of the gray-cheeked and in New Hampshire only near the tree line of the highest of the White Mountains. Its song has been described as a “double spiral” that ends in a jumble of notes.

All the thrushes produce two notes simultaneously, but with varying amounts of drama. If you listen closely to robins, some of the better singers among them approach the beauty of their Catharus cousins. So, while the more accomplished singers retain a juvenile appearance, their shared ancestry is apparent not only in the speckled young of robins and bluebirds, but also in their possession of an exquisite double-timbred syrinx.

Bill Chaisson, who has been a birdwatcher since age 11, is a former editor of the Eagle Times. He now works for the Town of Wilmot and lives in Sutton.